In general, fermentation products are produced by degradation of starch-containing material into fermentable sugars by liquefaction and saccharification followed by conversion of the sugars directly or indirectly into the desired fermentation product using a fermenting organism.
However, the industry of producing fermentation products such as ethanol and lactic acid is facing the challenge of redirecting the production process from fermentation of relatively easily convertible but expensive starchy materials, to the complex but inexpensive lignocellulosic biomass such as plant biomass.
Unlike starch, which contains homogenous and easily hydrolysed polymers, lignocellulosic biomass contains variable amounts of cellulose, hemicellulose, lignin and small amounts of protein, pectin, wax and other organic compounds. Lignocellulosic biomass should be understood in its broadest sense, so that it apart from wood, agricultural residues, energy crops also comprises different types of waste from both industry and households. Cellulosic biomass is a vast poorly exploited resource, and in some cases a waste problem. However, hexoses from cellulose can be converted by yeast to fuel ethanol for which there is a growing demand. Pentoses from hemicellulose cannot yet be converted to ethanol commercially but several promising ethanologenic microorganisms with the capacity to convert pentoses and hexoses are under development.
Typically, the first step in utilization of lignocellulosic biomass is a pre-treatment step, in order to fractionate the components of lignocellulosic material and increase their surface area.
The pre-treatment method most often used is steam pretreatment, a process comprising heating of the lignocellulosic material by steam injection to a temperature of 130-230° C. Prior to or during steam pretreatment, a catalyst like mineral or organic acid or a caustic agent facilitating disintegration of the biomass structure can be added optionally.
Another type of lignocellulose hydrolysis is acid hydrolysis, where the lignocellulosic material is subjected to an acid such as sulphuric acid whereby the sugar polymers cellulose and hemicellulose are partly or completely hydrolysed to their constituent sugar monomers and the structure of the biomass is destroyed facilitating access of hydrolytic enzymes in subsequent processing steps.
A further method is wet oxidation wherein the material is treated with oxygen at 150-185° C. Either pretreatment can be followed by enzymatic hydrolysis to complete the release of sugar monomers. This pre-treatment step results in the hydrolysis of cellulose into glucose while hemicellulose is transformed into the pentoses xylose and arabinose and the hexoses glucose, mannose and galactose. Thus, in contrast to starch, the hydrolysis of lignocellulosic biomass results in the release of pentose sugars in addition to hexose sugars. This implies that useful fermenting organisms need to be able to convert both hexose and pentose sugars to desired fermentation products such as ethanol.
After the pre-treatment the lignocellulosic biomass processing schemes involving enzymatic or microbial hydrolysis commonly involve four biologically mediated transformations: (1) the production of saccharolytic enzymes (cellulases and hemicellulases); (2) the hydrolysis of carbohydrate components present in pretreated biomass to sugars; (3) the fermentation of hexose sugars (e.g. glucose, mannose, and galactose); and (4) the fermentation of pentose sugars (e.g., xylose and arabinose).
Each processing step can make the overall process more costly and, therefore, decrease the economic feasibility of producing biofuel or carbon-based chemicals from cellulosic biological material. Thus, there is a need to develop methods that reduce the number of processing steps needed to convert cellulosic biological material to biofuel and other commercially desirable materials.
The four biologically mediated transformations may occur in a single step in a process configuration called consolidated bioprocessing (CBP), which is distinguished from other less highly integrated configurations in that CBP does not involve a dedicated process step for cellulase and/or hemicellulase production. CBP offers the potential for higher efficiency than a processes requiring dedicated cellulase production in a distinct unit operation.
Therefore, the availability of novel microorganisms for converting lignocellulosic biomass material to carbon-based chemicals would be highly advantageous.